Affinity chromatography has been used successfully in the isolation of alpha 2 u from liver, serum, kidney, and urine. Although the antiserum was produced against a single isoelectric species (pH 5.1) of alpha 2 u, all other isoelectric species reacted. Liver, serum, and kidney yielded species ranging in isoelectric point from 4.7 to 5.35; urine differed in that its species ranged from 5.6 to 6.3. All species regardless of source were of the same molecular weight, namely 16,000. The biosynthesis of alpha 2 u in the liver was followed from the rough through the smooth endoplasmic reticulum, and the Golgi apparatus to the blood plasma. Approximately 30 min were required for the transition. Excretion of alpha 2 u, albumin, and TUP was studied in a group of rats from date of weaning to 365 days of age. In the young adult the TUP was 30% sex-dependent protein (alpha 2 u) while after 365 days the principal protein excreted was serum albumin. BIBLIOGRAPHIC REFERENCES: Neuhas, O.W. and Flory, W., "The Effect of Dietary Protein on the Excretion of alpha 2 u, the Sex-Dependent Protein of the Adult Male Rat", Biochim. Biophys. Acta 411, 74-86 (1975).